U.S. patent number 8,737,544 [Application Number 13/598,184] was granted by the patent office on 2014-05-27 for method and apparatus of estimating frequency offset based on partial periodogram in wireless communication system.
This patent grant is currently assigned to Research & Business Foundation Sungkyunkwan University. The grantee listed for this patent is Dahae Chong, Seung Goo Kang, Junhwan Kim, Youngpo Lee, Seokho Yoon. Invention is credited to Dahae Chong, Seung Goo Kang, Junhwan Kim, Youngpo Lee, Seokho Yoon.
United States Patent |
8,737,544 |
Yoon , et al. |
May 27, 2014 |
Method and apparatus of estimating frequency offset based on
partial periodogram in wireless communication system
Abstract
A method and an apparatus of estimating a frequency offset in a
wireless communication system are provided. An orthogonal frequency
division multiplexing (OFDM) receiver performs envelope equalized
processing (EEP) with respect to a reception signal and calculates
a partial periodogram with a plurality of test values based on the
reception signal which goes through the EEP. The OFDM receiver
estimates a first frequency offset, a second frequency offset, and
a third frequency offset based on two partial periodograms adjacent
to each other among the partial periodograms for the plurality of
test values.
Inventors: |
Yoon; Seokho (Suwon-si,
KR), Chong; Dahae (Suwon-si, KR), Kim;
Junhwan (Suwon-si, KR), Kang; Seung Goo
(Suwon-si, KR), Lee; Youngpo (Suwon-si,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Yoon; Seokho
Chong; Dahae
Kim; Junhwan
Kang; Seung Goo
Lee; Youngpo |
Suwon-si
Suwon-si
Suwon-si
Suwon-si
Suwon-si |
N/A
N/A
N/A
N/A
N/A |
KR
KR
KR
KR
KR |
|
|
Assignee: |
Research & Business Foundation
Sungkyunkwan University (Suwon-si, KR)
|
Family
ID: |
47743704 |
Appl.
No.: |
13/598,184 |
Filed: |
August 29, 2012 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20130051447 A1 |
Feb 28, 2013 |
|
Foreign Application Priority Data
|
|
|
|
|
Aug 30, 2011 [KR] |
|
|
10-2011-0087120 |
|
Current U.S.
Class: |
375/344 |
Current CPC
Class: |
H04L
27/2675 (20130101); H04L 27/2657 (20130101); H04L
27/2672 (20130101) |
Current International
Class: |
H04L
27/06 (20060101) |
Field of
Search: |
;375/295,316,344,343 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Ren et al, "An Efficient Frequency Offset Estimation Method with
Large Rangefor Wireless OFDM Systems" IEEE Trans, Vechicular
Technology, vol. 56, pp. 1892-1895, Jul. 2007. cited by examiner
.
Lei et al, "Periodogram-based Carrier Frequency Offset Estimation
for Orthogonal Frequency Division Multiplexing Applications" IEEE
Global Telecommunications Conference, 2001 V5, pp. 3070-3074. cited
by examiner .
S. Kim, D. Chong, S. Y. Kim, and S. Yoon, "A novel
periodogram-based frequency offset estimation method for OFDM
systems," Proc. Int. Technical Conf. Circuits/Systems, Comp., and
Comm. (ITC-CSCC), pp. 97-100, Shimonoseki, Japan, Jul. 2008. cited
by examiner .
S. G. Kang, D. Chong, Y. Lee, and S. Yoon, "A robust
periodogram-based IFO estimation scheme for OFDM-based wireless
systems," Proc. Int. Conf. Commun. Theory, Reliability, and Quality
of Service (CTRQ), pp. 43-46, Budapest, Hungary, Apr. 2011. cited
by examiner .
Jan-Jaap van de Beek, et al. (Jul. 1997). "ML Estimation of Time
and Frequency Offset in OFDM Systems." IEEE Transactions on Signal
Processing, vol. 45, No. 7: pp. 1800-1805. cited by applicant .
Timothy M. Schmidl et al., (Dec. 1997). "Robust Frequency and
Timing Syncronization for OFDM." IEEE Transactions on
Communication. vol. 45, No. 12: pp. 1613-1621. cited by
applicant.
|
Primary Examiner: Joseph; Jaison
Attorney, Agent or Firm: NSIP Law
Claims
What is claimed is:
1. A method of estimating a frequency offset in a wireless
communication system, the method comprising: performing envelope
equalized processing (EEP) for a reception signal; calculating
partial periodograms for a plurality of test values based on the
reception signal which goes through the EEP; estimating a first
frequency offset based on two partial periodograms adjacent to each
other among the partial periodograms for the plurality of test
values; estimating a second frequency offset based on the estimated
first frequency offset and two partial periodograms adjacent to
each other based on the estimated first frequency offset;
repeatedly estimating a third frequency offset based on the
estimated first frequency offset, the estimated second frequency
offset, and two partial periodograms adjacent to each other based
on the estimated first frequency offset and the estimated second
frequency offset; and estimating a final frequency offset by adding
the estimated first frequency offset, the estimated second
frequency offset, and the estimated third frequency offset.
2. The method of claim 1, wherein: the partial periodograms are
calculated by the following equation,
.alpha..function..alpha..times..times..times..alpha..function..alpha..tim-
es..times.e.times..times..times..pi..times..times..times..times.
##EQU00025## where .alpha. denotes an interval among the plurality
of test values in a frequency domain, N denotes a length of
discrete Fourier transform (DFT), and y.sub.m denotes the reception
signal which goes through the EEP.
3. The method of claim 2, wherein: .alpha. is smaller than N and is
one of integers which are the power of 2.
4. The method of claim 2, wherein: the first frequency offset is
estimated as a test value when the sum of two adjacent partial
periodograms among the partial periodograms for the plurality of
test values has the maximum value.
5. The method of claim 4, wherein: the first frequency offset is
estimated by the following equation,
.epsilon..times..times..epsilon..times..alpha..function..epsilon..alpha..-
function..epsilon..alpha. ##EQU00026## where I.sub..alpha.({tilde
over (.epsilon.)}.sub.I) denotes the partial periodograms for the
test values and I.sub..alpha.({tilde over
(.epsilon.)}.sub.I+.alpha.) denotes a partial periodogram adjacent
to I.sub..alpha.({tilde over (.epsilon.)}.sub.I+.alpha.).
6. The method of claim 2, wherein: the second frequency offset is
estimated by the following equation,
.epsilon..alpha..times..alpha..function..epsilon..alpha..alpha..function.-
.epsilon..alpha..function..epsilon..alpha. ##EQU00027## where
I.sub..alpha.({circumflex over (.epsilon.)}.sub.I) denotes the
partial periodogram for the estimated first frequency offset and
I.sub..alpha.({circumflex over (.epsilon.)}.sub.I+.alpha.) denotes
the partial periodogram adjacent to I.sub..alpha.({circumflex over
(.epsilon.)}.sub.I).
7. The method of claim 2, wherein: the third frequency offset is
estimated by repeatedly computing the following equation,
.epsilon..alpha..times..alpha..function..epsilon..epsilon..epsilon..alpha-
..alpha..function..epsilon..epsilon..epsilon..alpha..times..alpha..functio-
n..epsilon..epsilon..epsilon..alpha..alpha..function..epsilon..epsilon..ep-
silon..alpha. ##EQU00028##
8. The method of claim 7, wherein: the equation is computed
repeatedly at log.sub.2 .alpha. times, and .alpha. is substituted
with .alpha./2 whenever the equation is computed repeatedly.
9. The method of claim 1, wherein: the EEP is performed by
multiplying the reception signal with a complex conjugate of a
training symbol and dividing the multiplication value by power of
the training symbol.
10. An orthogonal frequency division multiplexing (OFDM) receiver
in a wireless communication system, the OFDM receiver comprising: a
radio frequency (RF) unit configured to transmit or receive a radio
signal; and a process, operatively connected to the RF unit, and
configured for: performing envelope equalized processing (EEP) for
a reception signal, calculating a partial periodogram for a
plurality of test values based on the reception signal which goes
through the EEP, estimating a first frequency offset based on two
partial periodograms adjacent to each other among the partial
periodograms for the plurality of test values, estimating a second
frequency offset based on the estimated first frequency offset and
two partial periodograms adjacent to each other based on the
estimated first frequency offset, repeatedly estimating a third
frequency offset based on the estimated first frequency offset, the
estimated second frequency offset, and two partial periodograms
adjacent to each other based on the estimated first frequency
offset and the estimated second frequency offset, and estimating a
final frequency offset by adding the estimated first frequency
offset, the estimated second frequency offset, and the estimated
third frequency offset.
11. The OFDM receiver of claim 10, wherein: the partial
periodograms are calculated by the following equation,
.alpha..function..alpha..times..times..times..alpha..function..alpha..tim-
es..times.e.times..times..times..pi..times..times..times..times.
##EQU00029## where .alpha. denotes an interval among the plurality
of test values in a frequency domain, N denotes a length of
discrete Fourier transform (DFT), and y.sub.m denotes the reception
signal which goes through the EEP.
12. The OFDM receiver of claim 11, wherein: .alpha. is smaller than
N and is one of integers which are the power of 2.
13. The OFDM receiver of claim 11, wherein: the first frequency
offset is estimated as a test value when the sum of two adjacent
partial periodograms among the partial periodograms for the
plurality of test values has the maximum value.
14. The OFDM receiver of claim 13, wherein: the first frequency
offset is estimated by the following equation,
.epsilon..times..times..epsilon..times..alpha..function..epsilon..alpha..-
function..epsilon..alpha. ##EQU00030## where I.sub..alpha.({tilde
over (.epsilon.)}.sub.I) denotes the partial periodograms for the
test values and I.sub..alpha.({tilde over
(.epsilon.)}.sub.I+.alpha.) denotes a partial periodogram adjacent
to I.sub..alpha.({tilde over (.epsilon.)}.sub.I).
15. The OFDM receiver of claim 11, wherein: the second frequency
offset is estimated by the following equation,
.epsilon..alpha..times..alpha..function..epsilon..alpha..alpha..function.-
.epsilon..alpha..function..epsilon..alpha. ##EQU00031## where
I.sub..alpha.({circumflex over (.epsilon.)}.sub.I) denotes the
partial periodogram for the estimated first frequency offset and
I.sub..alpha.({circumflex over (.epsilon.)}.sub.I+.alpha.) denotes
the partial periodogram adjacent to I.sub..alpha.({circumflex over
(.epsilon.)}.sub.I).
16. The OFDM receiver of claim 11, wherein: the third frequency
offset is estimated by repeatedly computing the following equation,
.epsilon..alpha..times..alpha..function..epsilon..epsilon..epsilon..alpha-
..alpha..function..epsilon..epsilon..epsilon..alpha..times..alpha..functio-
n..epsilon..epsilon..epsilon..alpha..alpha..function..epsilon..epsilon..ep-
silon..alpha. ##EQU00032##
17. The OFDM receiver of claim 16, wherein: the equation is
computed repeatedly at log.sub.2 .alpha. times, and .alpha. is
substituted with .alpha./2 whenever the equation is computed
repeatedly.
18. The OFDM receiver of claim 10, wherein: the EEP is performed by
multiplying the reception signal with a complex conjugate of a
training symbol and dividing the multiplication value by power of
the training symbol.
19. A method of receiving an orthogonal frequency division
multiplexing (OFDM) signal in a wireless communication system, the
method comprising: adjusting frequency synchronization of an OFDM
reception signal; converting the OFDM reception signal in which the
time and the frequency are synchronized into a parallel signal;
performing fast Fourier transform (FFT) for the parallel signal;
and performing decoding and de-interleaving for the parallel signal
which goes through the FFT, wherein the adjusting of the frequency
synchronization of the OFDM reception signal includes: performing
envelope equalized processing (EEP) for the OFDM reception signal;
calculating a partial periodogram for a plurality of test values
based on the reception signal which goes through the EEP;
estimating a first frequency offset based on two partial
periodograms adjacent to each other among the partial periodograms
for the plurality of test values; estimating a second frequency
offset based on the estimated first frequency offset and two
partial periodograms adjacent to each other based on the estimated
first frequency offset; repeatedly estimating a third frequency
offset based on the estimated first frequency offset, the estimated
second frequency offset, and two partial periodograms adjacent to
each other based on the estimated first frequency offset and the
estimated second frequency offset; and estimating a final frequency
offset by adding the estimated first frequency offset, the
estimated second frequency offset, and the estimated third
frequency offset.
20. An apparatus of receiving an orthogonal frequency division
multiplexing (OFDM) signal in a wireless communication system, the
apparatus comprising: a synchronization block for adjusting
frequency synchronization of an OFDM reception signal; a
serial/parallel converter for converting the OFDM reception signal
in which the time and the frequency are synchronized into a
parallel signal; an FFT block for performing fast Fourier transform
(FFT) for the parallel signal; and a decoding/de-interleaving block
for performing decoding and de-interleaving for the parallel signal
which goes through the FFT, wherein the synchronization block is
configured for: performing envelope equalized processing (EEP) for
the OFDM reception signal; calculating a partial periodogram for a
plurality of test values based on the reception signal which goes
through the EEP; estimating a first frequency offset based on two
partial periodograms adjacent to each other among the partial
periodograms for the plurality of test values; estimating a second
frequency offset based on the estimated first frequency offset and
two partial periodograms adjacent to each other based on the
estimated first frequency offset; repeatedly estimating a third
frequency offset based on the estimated first frequency offset, the
estimated second frequency offset, and two partial periodograms
adjacent to each other based on the estimated first frequency
offset and the estimated second frequency offset; and estimating a
final frequency offset by adding the estimated first frequency
offset, the estimated second frequency offset, and the estimated
third frequency offset.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority of Korean Patent
application No. 10-2011-0087120 filed on Aug. 30, 2011, which is
incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communication, and more
particularly, to a method and an apparatus of estimating a
frequency offset based on a partial periodogram in a wireless
communication system.
2. Related Art
In the case of a broadband wireless communication system, effective
transmission/reception techniques and utilization schemes have been
proposed in order to maximize efficiency of limited wireless
resources. Next-generation wireless communication is an orthogonal
frequency division multiplexing (OFDM) system capable of reducing
an inter-symbol interference effect. In the OFDM, a data symbol
input in series is converted into N parallel data symbols, which
are transmitted with the N parallel data symbols being loaded on N
divided subcarriers, respectively. The subcarriers maintain
orthogonality in terms of a frequency. Respective orthogonal
channels go through inter-dependent frequency selective fading, and
as a result, complexity is reduced at a receiver and an interval of
the transmitted symbols is lengthened, thereby minimizing
inter-symbol interference. However, the OFDM system is very
sensitive to the frequency offset. The frequency offset may occur
by inconsistency in frequency of an oscillator between a
transmitter and a receiver or a Doppler effect. The orthogonality
among the subcarriers may be broken and interference may occur due
to the frequency offset, and as a result, demodulation performance
is reduced. This problem is commonly raised in all communication
systems using the OFDM technique, which includes an OFDM-based
cognitive radio (CR) system.
In order to estimate the frequency offset of the OFDM system,
various frequency offset estimating methods have been proposed.
First, a frequency offset estimating method based on two repeated
OFDM signals and a maximum likelihood theory may be proposed.
However, this method has a disadvantage that an estimate range of
the frequency offset is too small. T. M. Schmidl and D. C. Cox,
"Robust frequency and timing synchronization for OFDM," IEEE Trans.
Commun, vol. 45, no. 12, pp. 1613-1621, December 1997 proposes a
method of estimating the frequency offset by using a training
symbol having a repeated structure in one OFDM signal and a
training symbol configured by a pseudo noise code. The method of
estimating the frequency offset by using the training symbol is
excellent in estimation performance of the frequency offset, but
has a disadvantage that the frequency offset can be estimated only
when the OFDM signal is configured by a specified training symbol.
J.-J. van de Beek, M. Sandell, and P. O. Borjesson, "ML estimation
of time and frequency offset in OFDM systems," IEEE Trans. Sig.
Process., vol. 45, no. 7, pp. 1800-1805, July 1997 proposes a
blind-based frequency offset estimating method, but is not
excellent in estimation performance of the frequency offset.
As a result, a method of estimating the frequency offset based on a
periodogram may be proposed. The method of estimating the frequency
offset based on the periodogram provides a frequency offset
estimating method which can be applied to a predetermined training
symbol based on the envelope equalized processing (EEP) and the
periodogram.
However, the existing proposed method of estimating the frequency
offset based on the periodogram is too high in amount of
computation. As a result, a frequency offset estimating method
capable of lowering the amount of computation and complexity while
using the periodogram needs to be proposed.
SUMMARY OF THE INVENTION
The present invention provides a method and an apparatus of
estimating a frequency offset based on a partial periodogram in a
wireless communication system. In particular, the present invention
proposes a method and an apparatus of estimating the frequency
offset having the amount of computation and complexity which are
low by estimating the frequency offset by means of a partial
periodogram.
In an aspect, a method of estimating a frequency offset in a
wireless communication system is provided. The method includes
performing envelope equalized processing (EEP) for a reception
signal, calculating partial periodograms for a plurality of test
values based on the reception signal which goes through the EEP,
estimating a first frequency offset based on two partial
periodograms adjacent to each other among the partial periodograms
for the plurality of test values, estimating a second frequency
offset based on the estimated first frequency offset and two
partial periodograms adjacent to each other based on the estimated
first frequency offset, repeatedly estimating a third frequency
offset based on the estimated first frequency offset, the estimated
second frequency offset, and two partial periodograms adjacent to
each other based on the estimated first frequency offset and the
estimated second frequency offset, and estimating a final frequency
offset by adding the estimated first frequency offset, the
estimated second frequency offset, and the estimated third
frequency offset.
The partial periodograms may be calculated by the following
equation
.alpha..function..alpha..times..times..times..alpha..function..alpha..tim-
es..times.e.times..times..times..pi..times..times..times..times.
##EQU00001## where .alpha. denotes an interval among the plurality
of test values, N denotes a length of discrete Fourier transform
(DFT), and y.sub.m denotes the reception signal which goes through
the EEP.
.alpha. may be smaller than N and may be one of integers which are
the power of 2.
the first frequency offset may be estimated as a test value when
the sum of two adjacent partial periodograms among the partial
periodograms for the plurality of test values has the maximum
value.
The first frequency offset may be estimated by the following
equation
.epsilon..times..times..epsilon..times..alpha..function..epsilon..alpha..-
function..epsilon..alpha. ##EQU00002## where I.sub..alpha.({tilde
over (.epsilon.)}.sub.I) denotes the partial periodograms for the
test values and I.sub..alpha.({tilde over
(.epsilon.)}.sub.I+.alpha.) denotes a partial periodogram adjacent
to I.sub..alpha.({tilde over (.epsilon.)}.sub.I).
The second frequency offset may be estimated by the following
equation
.epsilon..alpha..times..alpha..function..epsilon..alpha..alpha..function.-
.epsilon..function..epsilon..alpha. ##EQU00003## where
I.sub..alpha.({circumflex over (.epsilon.)}.sub.I) denotes the
partial periodogram for the estimated first frequency offset and
I.sub..alpha.({circumflex over (.epsilon.)}.sub.I+.alpha.) denotes
the partial periodogram adjacent to I.sub..alpha.({circumflex over
(.epsilon.)}.sub.I).
The third frequency offset may be estimated by repeatedly computing
the following equation
.epsilon..alpha..times..alpha..function..epsilon..epsilon..epsilon..alpha-
..alpha..function..epsilon..epsilon..epsilon..alpha..times..alpha..functio-
n..epsilon..epsilon..epsilon..alpha..alpha..function..epsilon..epsilon..ep-
silon..alpha. ##EQU00004##
The equation may be computed repeatedly at log.sub.2 .alpha. times,
and .alpha. may be substituted with .alpha./2 whenever the equation
is computed repeatedly.
The EEP may be performed by multiplying the reception signal with a
complex conjugate of a training symbol and dividing the
multiplication value by power of the training symbol.
In another aspect, an orthogonal frequency division multiplexing
(OFDM) receiver in a wireless communication system is provided. The
OFDM receiver includes a radio frequency (RF) unit configured to
transmit or receive a radio signal, and a process, operatively
connected to the RF unit, and configured for performing envelope
equalized processing (EEP) for a reception signal, calculating a
partial periodogram for a plurality of test values based on the
reception signal which goes through the EEP, estimating a first
frequency offset based on two partial periodograms adjacent to each
other among the partial periodograms for the plurality of test
values, estimating a second frequency offset based on the estimated
first frequency offset and two partial periodograms adjacent to
each other based on the estimated first frequency offset,
repeatedly estimating a third frequency offset based on the
estimated first frequency offset, the estimated second frequency
offset, and two partial periodograms adjacent to each other based
on the estimated first frequency offset and the estimated second
frequency offset, and estimating a final frequency offset by adding
the estimated first frequency offset, the estimated second
frequency offset, and the estimated third frequency offset.
In another aspect, a method of receiving an orthogonal frequency
division multiplexing (OFDM) signal in a wireless communication
system is provided. The method includes adjusting frequency
synchronization of an OFDM reception signal, converting the OFDM
reception signal in which the time and the frequency are
synchronized into a parallel signal, performing fast Fourier
transform (FFT) for the parallel signal, and performing decoding
and de-interleaving for the parallel signal which goes through the
FFT, wherein the adjusting of the frequency synchronization of the
OFDM reception signal includes performing envelope equalized
processing (EEP) for the OFDM reception signal, calculating a
partial periodogram for a plurality of test values based on the
reception signal which goes through the EEP, estimating a first
frequency offset based on two partial periodograms adjacent to each
other among the partial periodograms for the plurality of test
values, estimating a second frequency offset based on the estimated
first frequency offset and two partial periodograms adjacent to
each other based on the estimated first frequency offset,
repeatedly estimating a third frequency offset based on the
estimated first frequency offset, the estimated second frequency
offset, and two partial periodograms adjacent to each other based
on the estimated first frequency offset and the estimated second
frequency offset, and estimating a final frequency offset by adding
the estimated first frequency offset, the estimated second
frequency offset, and the estimated third frequency offset.
In another aspect, an apparatus of receiving an orthogonal
frequency division multiplexing (OFDM) signal in a wireless
communication system is provided. The apparatus includes a
synchronization block for adjusting frequency synchronization of an
OFDM reception signal, a serial/parallel converter for converting
the OFDM reception signal in which the time and the frequency are
synchronized into a parallel signal, an FFT block for performing
fast Fourier transform (FFT) for the parallel signal, and a
decoding/de-interleaving block for performing decoding and
de-interleaving for the parallel signal which goes through the FFT,
wherein the synchronization block is configured for performing
envelope equalized processing (EEP) for the OFDM reception signal,
calculating a partial periodogram for a plurality of test values
based on the reception signal which goes through the EEP,
estimating a first frequency offset based on two partial
periodograms adjacent to each other among the partial periodograms
for the plurality of test values, estimating a second frequency
offset based on the estimated first frequency offset and two
partial periodograms adjacent to each other based on the estimated
first frequency offset, repeatedly estimating a third frequency
offset based on the estimated first frequency offset, the estimated
second frequency offset, and two partial periodograms adjacent to
each other based on the estimated first frequency offset and the
estimated second frequency offset, and estimating a final frequency
offset by adding the estimated first frequency offset, the
estimated second frequency offset, and the estimated third
frequency offset.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a general OFDM receiver.
FIG. 2 illustrates an exemplary embodiment of a proposed method of
estimating a frequency offset.
FIG. 3 illustrates another exemplary embodiment of a proposed
method of estimating a frequency offset.
FIG. 4 illustrates a difference in the amount of computation
depending on variation of a when the proposed frequency offset
estimating method is applied.
FIGS. 5 and 6 are graphs illustrating the performance of the
proposed frequency offset estimating method.
FIG. 7 is a block diagram of a wireless communication system in
which the exemplary embodiment of the present invention is
implemented.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
Hereinafter, exemplary embodiments of the present invention will be
described in detail so that those skilled in the art easily
implement the exemplary embodiments with reference to the
accompanying drawings. However, the present invention may be
implemented in various different forms and is not limited to the
exemplary embodiments described herein. Parts which are not
associated with the description are omitted in order to
specifically describe the present invention in the drawings and
like reference numerals refer to like elements throughout the
specification. Further, although the detailed description is
omitted, parts which can easily be understood by those skilled in
the art is not described.
Through the specification and the appended claims, when a
predetermined part "includes" a predetermined component, other
components are not excluded but other components may be further
included in the predetermined part.
FIG. 1 is a block diagram of a general OFDM receiver.
Referring to FIG. 1, the general OFDM receiver includes a radio
frequency local oscillator (RF LO) 10, an analog/digital converter
(A/D) 20, a serial/parallel converter (S/P) 30, a synchronization
block 40, an intermediate frequency local oscillator (IF LO) 50, a
fast Fourier transform (FFT) block 60, and a
decoding/de-interleaving block 70. A reception signal is input into
the S/P 30 and the synchronization block 40 through the A/D 20 and
an output of the synchronization block 40 is fed back to the IF LO
50 and input into the A/D 20 again. The synchronization block 40
synchronizes a time and a frequency. The reception signal passing
through the S/P 30 passes though the FFT block 60 and is output
through the decoding/de-interleaving block 70. The frequency offset
estimating method to be described below may be performed by the
synchronization block 40 in the block diagram of the OFDM receiver
of FIG. 1. That is, the performance of the synchronization block 40
of the OFDM receiver may be performed by the proposed frequency
offset estimating method.
Hereinafter, a method of estimating the frequency offset based on a
periodogram of the present invention will be described.
A bitstream is modulated by a phase shift keying (PSK) or
quadrature amplitude modulation (QAM) scheme, and as a result, a
data symbol is generated. Inverse fast Fourier transform (IFFT) is
performed with respect to the generated data symbol, and as a
result, an OFDM signal is generated in a time domain. The generated
OFDM signal may be expressed by Equation 1.
.times..times..times.e.times..times..times..pi..times..times..times..time-
s..times..times..times.<.times..times.> ##EQU00005##
In Equation 1, N represents the magnitude of the IFFT and X.sub.n
represents an n-th data symbol modulated in the PSK or QAM
scheme.
In a transmitter, a guard interval (GI) longer than a maximum delay
time of the channel may be inserted between the OFDM signals. This
is to remove interference between the OFDM signals, which may occur
due to the channel. In this case, the guard interval may be
inserted in a cyclic prefix (CP) form of the same form as a latter
part of the OFDM signal, in order to assure orthogonality between
subcarriers. When time synchronization is perfectly performed, a
k-th sample of the signal received through the channel may be
expressed by Equation 2.
.times..times..times.e.times..times..times..pi..times..times..times..time-
s..epsilon..times..times..times.<.times..times.>
##EQU00006##
In Equation 2, h.sub.1 represents a 1-th complex impulse response
coefficient of a channel having a length of L and .epsilon.
represents a frequency offset normalized at a subcarrier interval.
w.sub.n represents complex additive white Gaussian noise (AWGN)
having an average of 0 and a distribution of .sigma..sub.w.sup.2. A
signal-to-noise ratio (SNR) may be defined as
.sigma..sub.s.sup.2/.sigma..sub.w.sup.2 and
.sigma..sub.s.sup.2=E{|x.sub.n|.sub.2}.
A receiver may perform envelope equalized processing (EEP) with
respect to the reception signal. The EEP may equalize amplitudes of
all the reception signals. The EEP may be defined by Equation
3.
<.times..times.> ##EQU00007##
In Equation 3, x.sub.n* represents a complex conjugate of a
conjugate x.sub.n. The reception signal that goes through the EEP
may be expressed by Equation 4.
'.times..times..times..times..times.e.times..times..times..pi..epsilon..t-
imes..times..times..times..times..times.e.times..times..times..pi..epsilon-
..times..times..times..times..times..times.e.pi..times..times..epsilon..ti-
mes..times.'<.times..times.> ##EQU00008##
In Equation 4, w.sub.n' may be approximated to probability
variables of a normal distribution having an average of 0 by a
central limit theorem (CLM). The reception signal may be changed to
a simple complex tone signal by the EEP.
The frequency offset estimating method of the present invention may
be performed dividedly in 3 steps. That is, a frequency offset to
be estimated,
.epsilon.=.epsilon..sub.I+.epsilon..sub.F+.epsilon..sub.R, is
constituted by a first frequency offset .epsilon..sub.I, a second
frequency offset .epsilon..sub.F, and a third frequency offset
.epsilon..sub.R, ands may be acquired by estimating
.epsilon..sub.I, .epsilon..sub.F, and .epsilon..sub.R for each
step.
First, the first frequency offset .epsilon..sub.I is estimated. The
.epsilon..sub.I may be estimated by Equation 5.
.epsilon..times..times..epsilon..times..function..epsilon..function..epsi-
lon.<.times..times.> ##EQU00009##
In Equation 5,
.epsilon..di-elect cons..times..times..times. ##EQU00010## is a
test value for finding {circumflex over (.epsilon.)}.sub.I which is
an estimation value of the first frequency offset. A periodogram
I(z) may be expressed by Equation 6.
.function..times..times.e.times..times..times..pi..times..times..times..t-
imes.<.times..times.> ##EQU00011##
In Equation 5, since an interval of {tilde over (.epsilon.)}.sub.I
is defined as 1, the first frequency offset .epsilon..sub.I is
acquired by estimating an integer part in the frequency offset
.epsilon.. Therefore, the second frequency offset .epsilon..sub.F
and the third frequency offset .epsilon..sub.R are acquired by
estimating a decimal part in the frequency offset .epsilon..
Meanwhile, when the frequency offset of the integer part is
estimated by Equations 5 and 6, all integer values of the frequency
offset need to be examined. Therefore, the amount of computation
increases and complexity of the receiver increases. In order to
solve these problems, the first frequency offset .epsilon..sub.I
may be estimated by using the partial periodogram,
.alpha..function..alpha..times..times..times..alpha..function..alpha..tim-
es..times.e.times..times..times..pi..times..times..times..times.
##EQU00012## When the partial periodogram is used, .epsilon..sub.I
may be estimated by Equation 7.
.epsilon..times..times..epsilon..times..alpha..function..epsilon..alpha..-
function..epsilon..alpha.<.times..times.> ##EQU00013##
In Equation 7,
.epsilon..di-elect cons..alpha..times..times..times..times..alpha.
##EQU00014## and the interval of {tilde over (.epsilon.)}.sub.I is
not 1 but .alpha.. .alpha. may be the power of 2 that is smaller
than N. Therefore, the number of examination times may be decreased
depending on .alpha. which is set.
Meanwhile, when there is no noise, Equation 7 has the large value
at {tilde over (.epsilon.)}.sub.I in the range of
.epsilon.-.alpha.<{tilde over
(.epsilon.)}.sub.I.ltoreq..epsilon.. Therefore, the second
frequency offset .epsilon..sub.F may be estimated in the range of
0.ltoreq..epsilon..sub.F.ltoreq..alpha..
When a single-path channel without noise is assumed,
I.sub..alpha.(.epsilon..sub.I) and
I.sub..alpha.(.epsilon..sub.I+.alpha.) may be approximated by
Taylor series as illustrated in Equations 8 and 9.
.alpha..function..epsilon..apprxeq..times..alpha..times..times..times..fu-
nction..epsilon..epsilon..alpha.<.times..times.>.alpha..function..ep-
silon..alpha..apprxeq..times..alpha..times..times..times..function..epsilo-
n..epsilon..alpha..alpha.<.times..times.> ##EQU00015##
Herein, sin c(x)=sin(.pi.x)/.pi.x. The relationship of Equation 10
may be deduced from Equations 8 and 9.
.alpha..function..epsilon..alpha..alpha..function..epsilon..times..times.-
.times..epsilon..epsilon..alpha..alpha..times..times..times..epsilon..epsi-
lon..alpha..epsilon..epsilon..epsilon..epsilon..alpha..times..times..epsil-
on..epsilon..alpha..alpha..times..epsilon..epsilon..alpha..epsilon..epsilo-
n..epsilon..epsilon..alpha.<.times..times.> ##EQU00016##
In this case, since .epsilon.-.epsilon..sub.I=.epsilon..sub.F,
Equation 10 may be expressed as Equation 11 again if
0.ltoreq..epsilon..sub.F.ltoreq..alpha..
.alpha..function..epsilon..alpha..alpha..function..epsilon..epsilon..epsi-
lon..alpha.<.times..times.> ##EQU00017##
The second frequency offset .epsilon..sub.F may be estimated by
Equation 12 based on Equation 11.
.epsilon..alpha..times..alpha..function..epsilon..alpha..alpha..function.-
.epsilon..alpha..function..epsilon..alpha.<.times..times.>
##EQU00018##
Meanwhile, when .epsilon..sub.F is close to 0 or a in Equation 12,
the SNR of any one of I.sub..alpha.({circumflex over
(.epsilon.)}.sub.I) and I.sub..alpha.({circumflex over
(.epsilon.)}.sub.I+.alpha.) decreases, and as a result, the
estimation performance of .epsilon..sub.F by Equation 12
deteriorates and the remaining frequency offsets need to be
additionally estimated. In this case, I.sub..alpha.({circumflex
over (.epsilon.)}.sub.I+{circumflex over
(.epsilon.)}.sub.F+.alpha./2) and I.sub..alpha.({circumflex over
(.epsilon.)}.sub.I+{circumflex over (.epsilon.)}.sub.F-.alpha./2)
using the estimation values of .epsilon..sub.I and .epsilon..sub.F
may have SNRs of an appropriate level which are not too low and the
relationship of Equation 13 may be deduced by using the same.
.alpha..function..epsilon..epsilon..alpha..alpha..function..epsilon..epsi-
lon..alpha..alpha..function..epsilon..epsilon..alpha..alpha..function..eps-
ilon..epsilon..alpha..epsilon..alpha..epsilon..alpha..epsilon..alpha..epsi-
lon..alpha.<.times..times.> ##EQU00019##
In Equation 13, .epsilon..sub.R=.epsilon.-{circumflex over
(.epsilon.)}.sub.I-{circumflex over (.epsilon.)}.sub.F and Equation
13 becomes 2.epsilon..sub.R/.alpha. in the range of
-.alpha./2<.epsilon..sub.R<.alpha./2. The third frequency
offset .epsilon..sub.R may be estimated based on Equation 14 by
using the same.
.epsilon..alpha..times..alpha..function..epsilon..epsilon..epsilon..alpha-
..times..times..alpha..function..epsilon..epsilon..epsilon..alpha..times..-
alpha..function..epsilon..epsilon..epsilon..alpha..times..times..alpha..fu-
nction..epsilon..epsilon..epsilon..alpha.<.times..times.>
##EQU00020##
In Equation 14, first, .epsilon..sub.T+1 is acquired by setting an
initial value as .epsilon..sub.T=0. New .epsilon..sub.T+1 is
acquired by substituting .epsilon..sub.T with .epsilon..sub.T+1
acquired above and .alpha. with .alpha./2. Such computation is
repeated until .alpha.=1. .epsilon..sub.T+1 which is finally
acquired becomes the estimation value of the third frequency offset
.epsilon..sub.R. In this case, since the number of repetition times
of the computation of Equation 14 increases as a increases, the
total amount of computation may increase, but the amount of
computation decreases very significantly while a decreases to a
half, and as a result, the total amount of computation makes no
odds.
A total frequency offset may be estimated by adding all the
estimation values of the first frequency offset .epsilon..sub.I,
the second frequency offset .epsilon..sub.F, and the third
frequency offset .epsilon..sub.R.
FIG. 2 illustrates an exemplary embodiment of a proposed method of
estimating a frequency offset.
In step S100, the OFDM receiver performs the EEP with respect to
the reception signal. In step S110 (first step), the OFDM receiver
estimates the first frequency offset .epsilon..sub.I based on
Equation 7 with respect to the reception signal which goes through
the EEP. In step S120 (second step), the OFDM receiver estimates
the second frequency offset .epsilon..sub.F based on Equation 12 by
using the partial periodogram used to acquire the estimation value
of .epsilon..sub.I. In step S130 (third step), the OFDM receiver
estimates the third frequency offset .epsilon..sub.R based on the
estimation value of .epsilon..sub.I, the estimation value of
.epsilon..sub.F, and Equation 14. An estimation value of a final
frequency offset may be acquired by adding all the estimation
values of the first frequency offset, the second frequency offset,
and the third frequency offset.
FIG. 3 illustrates another exemplary embodiment of a proposed
method of estimating a frequency offset.
In step S200, the OFDM receiver performs the EEP with respect to
the reception signal. In step S210, the OFDM receiver calculates
the partial periodograms for a plurality of test values based on
the reception signal which goes through the EEP. In step S220, the
OFDM receiver estimates the first frequency offset based on two
partial periodograms adjacent to each other among the partial
periodograms for the plurality of test values. In this case,
Equation 7 may be used. In step S230, the OFDM receiver estimates
the second frequency offset based on the estimated first frequency
offset and two partial periodograms adjacent to each other based on
the estimated first frequency offset. In this case, Equation 12 may
be used. In step S240, the OFDM receiver repeatedly estimates the
third frequency offset based on the estimated first frequency
offset, the estimated second frequency offset, and two partial
periodograms adjacent to each other based on the estimated first
frequency offset and the estimated second frequency offset. In this
case, Equation 14 may be used. In step S250, the OFDM receiver
estimates the final frequency offset by adding the estimated first
frequency offset, the estimated second frequency offset, and the
estimated third frequency offset.
Table 1 illustrates the amount of computation required in each of
the steps of estimating the first frequency offset, the second
frequency offset, and the third frequency offset. Since the
multiplication of conjugates may be expressed by the multiplication
of four real numbers and the sum of two real numbers and the sum of
conjugates may be expressed by the sum of two real numbers, the
total amount of computation may be acquired according to the amount
of computation when the multiplication of the real numbers is
calculated and the amount of computation when the sum of the real
numbers is calculated.
TABLE-US-00001 TABLE 1 Multiplication of real numbers Sum of real
numbers EEP 4N 2N First step .times..alpha..times..alpha.
##EQU00021## .alpha..times..times..alpha. ##EQU00022## Second step
2 1 Third step
.function..alpha..times..times..alpha..times..function..alpha.
##EQU00023##
.function..alpha..times..times..times..alpha..function..function..alpha.
##EQU00024##
FIG. 4 illustrates a difference in the amount of computation
depending on variation of .alpha. when the proposed frequency
offset estimating method is applied. FIG. 4 illustrates a
difference in the amount of computation depending on the variation
of .alpha. when N=64. In general, it can be seen that the amount of
computation decreases as a increases. However, the amount of
computation increases slightly when .alpha.=32 as compared with
.alpha.=16. The reason is that the larger amount of computation is
increased by repeating the third step than the amount of
computation which decreases in the first step.
FIGS. 5 and 6 are graphs illustrating the performance of the
proposed frequency offset estimating method.
FIG. 5 illustrates mean square error (MSE) performance of the
proposed frequency offset estimating method depending on .alpha.
when a training symbol of an institute of electrical and
electronics engineers (IEEE) 802.11a standard is used. FIG. 6
illustrates MSE performance of the proposed frequency offset
estimating method depending on .alpha. when a training symbol of an
IEEE 802.16-2004 standard is used. Ns in FIGS. 5 and 6 are 64 and
256, respectively. As a channel model, a 4-path Rayleigh fading
channel is used, each channel response has a time delay of 0, 2, 4,
or 6 sample in the Rayleigh fading channel, power of an 1-th
impulse response of the channel, h.sub.1.sup.2 is exponentially
decreased as 1 increases like E{h.sub.1.sup.2}=exp(-0.81). A
Doppler bandwidth is set to 0.0017, which corresponds to a case in
which a movement velocity is 120 km/h.
Referring to FIGS. 5 and 6, in general, the MSE performance
decreases as .alpha. increases. The reason is that since the value
of the partial periodogram is configured by adding .alpha. values
which go through computation of an absolute value, the amount of
noise increases as a increases. However, the MSE performance does
not significantly decrease until .alpha.=4 in FIG. 5 and .alpha.=8
in FIG. 6. Although a increases, when the SNR is 18 dB or more in
FIG. 5 and the SNR is 21 dB or more in FIG. 6, the MSE performance
does not decrease. Therefore, in the IEEE 802.11a system of FIG. 5,
the frequency offset may be estimated with optimal performance when
.alpha.=4 and in the IEEE 802.16-2004 system of FIG. 6, the
frequency offset may be estimated with optimal performance when
.alpha.=8. When the SNR is known, the frequency offset estimating
method with reliability may be acquired while the amount of
computation decreases by selecting .alpha. according to the
SNR.
FIG. 7 is a block diagram of a wireless communication system in
which the exemplary embodiment of the present invention is
implemented.
A transmitter 800 includes a processor 810, a memory 820, and a
radio frequency (RF) unit 830. The processor 810 implements the
proposed functions, processes, and/or methods. Layers of radio
interface protocols may be implemented by the processor 810. The
memory 820 is connected with the processor 810 to store various
information for driving the processor 810. The RF unit 830 is
connected with the processor 810 to transmit and/or receive a radio
signal.
A receiver 900 includes a processor 910, a memory 920, and an RF
unit 930. The processor 910 implements the proposed functions,
processes, and/or methods. The layers of the radio interface
protocols may be implemented by the processor 910. The memory 920
is connected with the processor 910 to store various information
for driving the processor 910. The RF unit 930 is connected with
the processor 910 to transmit and/or receive the radio signal.
The processors 810 and 910 may include an application-specific
integrated circuit (ASIC), other chipsets, a logic circuit, and/or
a data processing device. The memories 820 and 920 may include a
read-only memory (ROM), a random access memory (RAM), a flash
memory, a memory card, a storage medium and/or other storage
devices. The RF units 830 and 930 may include a baseband circuit
for processing the radio signal. When the exemplary embodiment is
implemented by software, the above-mentioned technique may be
implemented by modules (a process and a function) performing the
above-mentioned functions. The module is stored in the memories 820
and 920 and may be executed by the processor 810 and 910. The
memories 820 and 920 may be provided inside or outside the
processors 810 and 910 and may be connected with the processor 810
and 910 by various well-known means.
The frequency offset may be estimated with excellent performance
under various noise environments by the proposed frequency offset
estimating method.
According to the exemplary embodiments of the present invention,
the frequency offset can be estimated with reliability while
lowering the amount of computation by using the partial
periodogram.
In the aforementioned exemplary system, the methods are described
based on a flowchart as a series steps or blocks, but the present
invention is not limited to the order of the steps and a
predetermined step may be performed in the different order from and
at the same time as other steps. Further, it may be appreciated by
those skilled in the art that steps shown in a flow chart is
non-exclusive and therefore, include other steps or deletes one or
more steps of a flowchart without having an effect on the scope of
the present invention.
* * * * *